1. Field of the Invention
The present invention pertains to combined extrusion and injection processing systems that convert powder and liquid mixtures into molded products. More particularly, the combined extrusion and injection processing systems are used to make molded food products.
2. Description of the Related Art
Extrusion systems that are used in the plastics industry commonly operate on stable plastic pellets that are fed into the screw or screws in the extruder barrel. The turning screws typically shear and heat the pellets, along with other materials, to form a modified plastic in melt form, which is forced through a die to form a wide variety of plastic products, including materials for use in injection molding. In the case of plastics, injection molding is accomplished by injecting plastic into a mold and subjecting it to heat and pressure, thereby producing a plastic object that mirrors the shape of the mold.
Extrusion and injection molding are commonly carried out in different machines at different times. However, there exists a class of unitary machines that perform extrusion and injection molding in one pass. The extrusion section of the machine takes in plastic pellets, and optional compounds if desired, melts them by the shearing and heating action of the screws, and feeds the resultant melt directly into the receiving facility of the injection-molding section of the machine. The melt is then injected into a mold pursuant to the usual procedures and formed into a desired shape.
In the plastics industry, the pelletized feed source is generally prepared in a separate extrusion process, either with or without compounding. This two-step process allows for the production of plastic pellets by a chemical company or compounders, who may supply the pellets to a manufacturer of plastic products. Relatively speaking, the pellets are chemically and physically stable. They are eminently suitable for use in conventional injection molding machines.
This is rarely true for most food products. First, it is not always possible to make pellets that can be easily handled for later injection molding. Second, where pellets can be made, they are not typically chemically or physically stable. Among other things, they age and dry out. Care must be taken in their packaging, storage environment and transportation, both to and on site. Failure to do so can render the pellets difficult or impossible to melt in an extruder. Beyond this, food pellets are sensitive to even small deviations in extrusion conditions. For example, the amount of liquid in the formulation may be such that the pellets either do not form cohesively or form into a monolithic mass that is difficult to process. Excessive pressures develop in the extruder, and the food mixture can be easily overworked. In short, pelletizing of foods, especially foods containing flour or starch, is usually not practical or cost effective.
The problems discussed above with reference to pelletized feeds, as well as the formation of excessive pressures and overworking of food ingredients, significantly if not completely limits the utility of unitary extrusion-molding systems in the food industry. This is disappointment in view of the benefits offered by these systems in other industries, including the plastics industries.